In many industries, such as the aerospace industry, one of the effective ways to reduce weight of the aircraft is to reduce the density of aluminum alloys used in the aircraft's construction. It is known in the art that aluminum alloy densities may be reduced by the addition of lithium but the addition of lithium to aluminum based alloys also raises other problems. For example, the addition of lithium to aluminum alloys may result in a decrease in ductility and fracture toughness. For use as aircraft structural parts, it is obviously imperative that any alloy have excellent fracture toughness and strength properties. It will be appreciated that both high strength and high fracture toughness are difficult to obtain in conventional alloys normally used in aircraft applications. See, for example, the publication by J. T. Staley entitled "Microstructure and Toughness of High Strength Aluminum Alloy, Properties Related to Fracture Toughness," ASTM STP 605, American Society for Testing and Materials, 1976, pages 71-103, which suggests generally that for sheet formed from the Alloy AA2024, toughness decreases as strength increases. The same has been observed to be true for the Alloy AA7050. A more desirable alloy would permit increased strength with only minimal or no decrease in fracture toughness or would permit processing steps wherein the fracture toughness was controlled as the strength was increased in order to provide a more desirable combination of strength and fracture toughness. Such alloys would find a widespread use in the aerospace industry where low density and high strength together with fracture toughness are highly desired.
Aluminum alloys are currently applied in high performance aircraft in peak strength or over aged heat treat conditions. They do not show degradation in fatigue, fracture or corrosion properties with exposure to thermal cycles usually encountered in parts such as bulkheads located near inlets and engine bays. Commercially available aluminum-lithium alloys such as AA2090, AA2091 and AA8090, have demonstrated a good combination of strength and fracture toughness but only in underaged conditions. In these alloys, fracture toughness is at a minimum in the peak strength condition and does not increase with overaging as with conventional alloys. Thus, the alloys are considered unstable with respect to thermal exposure. Short transverse fracture toughness for even an underaged condition, typically sixteen ksi .sqroot.in in AA8090, is well below minimum requirements for conventional alloys and considered to be too low for most applications. Also, like Alloy AA2124, the underaged conditions of Alloy AA2090 have demonstrated susceptibility to stress corrosion cracking (SCC) while the peak strength condition is resistent to stress corrosion cracking. Alloy AA2024 is an aluminum based alloy containing 3.8-4.9 weight percent copper, 1.2-1.8 weight percent magnesium, 0.30-0.9 weight percent manganese and a nominal copper to magnesium atomic ratio of 1.1 with a density of 0.101 pounds per cubic inch and a peak tensile yield strength (TYS) of 67 ksi. Alloy AA2090 is an aluminum based alloy containing 1.9-2.6 weight percent lithium, 2.4-3.0 weight percent copper, 0.25 maximum weight percent magnesium, 0.05 maximum weight percent manganese, with a nominal density of 0.0940 pounds per cubic inch and a TYS of 71 ksi. Alloy AA8090 is an aluminum based alloy containing 2.2-2.7 weight percent lithium, 1.0-1.6 weight percent copper, 0.6-1.3 weight percent magnesium, a maximum of 0.10 weight percent manganese, a maximum of 0.10 weight percent chromium, a maximum of 0.25 weight percent zinc, a maximum of 0.10 weight percent titanium and 0.04-0.16 weight percent zirconium, with a copper to magnesium atomic ratio of 0.7, a nominal density of 0.092 pounds per cubic inch and a TYS of 59 ksi. All percentages are weight percentages unless otherwise indicated.
There are many disclosures in the prior art of aluminum based alloys which contain lithium, copper and sometimes magnesium and manganese. Thus, U.S. Pat. No. 4,840,682 discloses in column 3 a table listing aluminum alloys which contain varying amounts of lithium, magnesium, copper, zirconium, manganese and minor amounts of other materials. In the actual example in this patent, the alloy contains 2.4 percent lithium, 1 percent magnesium, 1.3 percent copper and 0.15 percent zirconium, with the balance aluminum.
U.S. Pat. No. 4,889,569 discloses in a table in column 3 alloys of various compositions. In the actual patent examples, lithium appears to always be 2.0 percent and copper is 2.2 percent.
French Patent No. 2,561,261, EPO 158571 and U.S. Pat. No. 4,752,343, which appear to be directed to the same alloys, disclose alloys which contain varying amounts of lithium, copper, magnesium, iron, silicon and other elements. Generally, lithium is said to range from 1.7 to 2.9 percent, copper from 1.5 to 3.4 percent and magnesium from 1.2 to 2.7 percent but with limitations on the magnesium/copper ratio.
German Patent No. 3,346,882 and British 2,134,929 show at Table 1 a series of aluminum based lithium alloys which contain copper, magnesium and other ingredients.
U.S. Pat. No. 4,648,943 discloses an aluminum based alloy wrought product wherein, in the working examples, the aluminum alloy contains 2.0 percent lithium, 2.7 percent copper, 0.65 percent magnesium and 0.12 percent zirconium.
U.S. Pat. No. 4,636,357 discloses an aluminum alloy in which the lithium component ranges from 2.2 to 3.0 percent with a small amount of copper but a substantial amount of zinc.
U.S. Pat. No. 4,624,717 discloses an aluminum based alloy wherein the lithium component is about 2.3 to 2.9 percent and the copper component is 1.6 to 2.4 percent.